In a radio system such as a terrestrial digital television broadcasting, a wireless LAN, or IEEE802.16, there has been used an OFDM (orthogonal frequency division multiplex) signal. The OFDM signal is a signal in which symbols are transmitted in parallel by the aid of a plurality of carrier waves that are different in the center frequency from each other. Also, the signals are transmitted orthogonally to each other to prevent the bands of the adjacent carrier waves from interfering with each other. In general, there arises such a problem that the OFDM signal is larger in peak amplitude than an average signal level, that is, the RMS value (root-mean-square of signal amplitude) of the signal. The ratio of the peak amplitude to the RMS value is called “PAPR” (peak to average power ratio). A circuit that deals with a signal having a large PAPR is required to have a wide dynamic range. Accordingly, in particular, it is necessary to set the operating point of a radio amplifier at a final stage of the transmitting device to be low, as a result of which the power efficiency of the final radio amplifier is deteriorated. The same problem also arises in a communication device using a CDMA (code division multiple access). Therefore, there has been known a technique by which the maximum level of the signal amplitude is reduced before the radio signal is amplified.
JP 2003-124824 A discloses a technique in which the peak (signal portion having a large amplitude) of a transmission signal is detected, and a given signal waveform having the same band as that of a transmission signal spectrum is subtracted from the transmission signal according to the peak of the transmission signal, to thereby suppress the peak of the transmission signal.
Also, US 2004/0218689 discloses a technique in which the peak of a transmission signal which exceeds a given threshold value is detected, and a signal c[m] resulting from allowing a pulse signal train p[m] produced based on the detected peak to pass through a filter is subtracted from the transmission signal, to thereby suppress the peak of the transmission signal. US 2004/0218689 also discloses a technique in which the filter that inputs the pulse signal train p[m] is configured in such a manner that the frequency response and the impulse response of the filter are configured so that the frequency components of the signal band of the transmission signal spectrum and the frequency components outside the signal (frequency) band pass through the filter at a given ratio, and so that the signal c[m] that has passed through the filter includes the frequency components within the signal band and the frequency components outside the signal band.